Inverters are widely used in power electronics applications in conversion between direct current (DC) power and alternating current (AC) power. The most common inverter type, PWM-inverter, converts a DC voltage into an AC output voltage, which consists of pulses with varying widths. The method by which the output voltage is formed is called pulse width modulation (PWM), and the objective in the output voltage forming is normally to create a pulse pattern with a desired fundamental component and minimum content of disadvantageous harmonics.
At low voltage (e.g. grid voltage less than 1 kV) the most common inverter type is a two-level inverter, by which the output voltage can have only values of the positive pole or of the negative pole of the supplying DC voltage. At higher grid voltages, multilevel inverters are often used, due to e.g. the voltage withstand limitations of commercial power components and better waveform of the created output voltage.
Medium grid voltage level (e.g. grid voltage more than 1 kV) is often used in industrial applications. At this voltage level, inverters capable to create output voltages with 3 to 5 level steps are sometimes provided, in part because the inverter power stage is still possible to realize by using commercially available IGBT (insulated gate bipolar transistor) components. A number of known topologies exist for the purpose, e.g. NPC (neutral point clamped), MMC (modular multilevel converter) and CHB (cascaded H-bridge), to name a few.
Drawbacks of the known topologies are e.g. high voltage steps, especially by NPC having a 3-level output voltage, which tend to cause harmful overvoltage spikes at the motor end of a long cable, thus reducing the lifetime of winding insulations. By some topologies the voltage withstand capability of power components limits the usability. Further, by MMC and CHB the component number and complexity of the control arrangement may cause a disadvantage from the system cost point of view.